/*
 * Copyright (C) 2016 The Guava Authors
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 * http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package com.google.common.graph;

import static com.google.common.base.Preconditions.checkNotNull;
import static com.google.common.base.Preconditions.checkState;

import com.google.common.annotations.Beta;
import com.google.common.base.MoreObjects;
import com.google.common.base.MoreObjects.ToStringHelper;
import com.google.common.base.Objects;
import com.google.common.collect.Maps;
import com.google.common.collect.Ordering;
import com.google.errorprone.annotations.Immutable;

import java.util.Comparator;
import java.util.Map;
import javax.annotation.CheckForNull;

/**
 * Used to represent the order of elements in a data structure that supports different options for
 * iteration order guarantees.
 *
 * <p>Example usage:
 *
 * <pre>{@code
 * MutableGraph<Integer> graph =
 *     GraphBuilder.directed().nodeOrder(ElementOrder.<Integer>natural()).build();
 * }</pre>
 *
 * @author Joshua O'Madadhain
 * @since 20.0
 */
@Beta
@Immutable
@ElementTypesAreNonnullByDefault
public final class ElementOrder<T>
{
    private final Type type;

    @SuppressWarnings("Immutable") // Hopefully the comparator provided is immutable!
    @CheckForNull
    private final Comparator<T> comparator;

    /**
     * The type of ordering that this object specifies.
     *
     * <ul>
     *   <li>UNORDERED: no order is guaranteed.
     *   <li>STABLE: ordering is guaranteed to follow a pattern that won't change between releases.
     *       Some methods may have stronger guarantees.
     *   <li>INSERTION: insertion ordering is guaranteed.
     *   <li>SORTED: ordering according to a supplied comparator is guaranteed.
     * </ul>
     */
    public enum Type
    {
        UNORDERED,
        STABLE,
        INSERTION,
        SORTED
    }

    private ElementOrder(Type type, @CheckForNull Comparator<T> comparator)
    {
        this.type = checkNotNull(type);
        this.comparator = comparator;
        checkState((type == Type.SORTED) == (comparator != null));
    }

    /**
     * Returns an instance which specifies that no ordering is guaranteed.
     */
    public static <S> ElementOrder<S> unordered()
    {
        return new ElementOrder<>(Type.UNORDERED, null);
    }

    /**
     * Returns an instance which specifies that ordering is guaranteed to be always be the same across
     * iterations, and across releases. Some methods may have stronger guarantees.
     *
     * <p>This instance is only useful in combination with {@code incidentEdgeOrder}, e.g. {@code
     * graphBuilder.incidentEdgeOrder(ElementOrder.stable())}.
     *
     * <h3>In combination with {@code incidentEdgeOrder}</h3>
     *
     * <p>{@code incidentEdgeOrder(ElementOrder.stable())} guarantees the ordering of the returned
     * collections of the following methods:
     *
     * <ul>
     *   <li>For {@link Graph} and {@link ValueGraph}:
     *       <ul>
     *         <li>{@code edges()}: Stable order
     *         <li>{@code adjacentNodes(node)}: Connecting edge insertion order
     *         <li>{@code predecessors(node)}: Connecting edge insertion order
     *         <li>{@code successors(node)}: Connecting edge insertion order
     *         <li>{@code incidentEdges(node)}: Edge insertion order
     *       </ul>
     *   <li>For {@link Network}:
     *       <ul>
     *         <li>{@code adjacentNodes(node)}: Stable order
     *         <li>{@code predecessors(node)}: Connecting edge insertion order
     *         <li>{@code successors(node)}: Connecting edge insertion order
     *         <li>{@code incidentEdges(node)}: Stable order
     *         <li>{@code inEdges(node)}: Edge insertion order
     *         <li>{@code outEdges(node)}: Edge insertion order
     *         <li>{@code adjacentEdges(edge)}: Stable order
     *         <li>{@code edgesConnecting(nodeU, nodeV)}: Edge insertion order
     *       </ul>
     * </ul>
     *
     * @since 29.0
     */
    public static <S> ElementOrder<S> stable()
    {
        return new ElementOrder<>(Type.STABLE, null);
    }

    /**
     * Returns an instance which specifies that insertion ordering is guaranteed.
     */
    public static <S> ElementOrder<S> insertion()
    {
        return new ElementOrder<>(Type.INSERTION, null);
    }

    /**
     * Returns an instance which specifies that the natural ordering of the elements is guaranteed.
     */
    public static <S extends Comparable<? super S>> ElementOrder<S> natural()
    {
        return new ElementOrder<>(Type.SORTED, Ordering.<S>natural());
    }

    /**
     * Returns an instance which specifies that the ordering of the elements is guaranteed to be
     * determined by {@code comparator}.
     */
    public static <S> ElementOrder<S> sorted(Comparator<S> comparator)
    {
        return new ElementOrder<>(Type.SORTED, checkNotNull(comparator));
    }

    /**
     * Returns the type of ordering used.
     */
    public Type type()
    {
        return type;
    }

    /**
     * Returns the {@link Comparator} used.
     *
     * @throws UnsupportedOperationException if comparator is not defined
     */
    public Comparator<T> comparator()
    {
        if (comparator != null)
        {
            return comparator;
        }
        throw new UnsupportedOperationException("This ordering does not define a comparator.");
    }

    @Override
    public boolean equals(@CheckForNull Object obj)
    {
        if (obj == this)
        {
            return true;
        }
        if (!(obj instanceof ElementOrder))
        {
            return false;
        }

        ElementOrder<?> other = (ElementOrder<?>) obj;
        return (type == other.type) && Objects.equal(comparator, other.comparator);
    }

    @Override
    public int hashCode()
    {
        return Objects.hashCode(type, comparator);
    }

    @Override
    public String toString()
    {
        ToStringHelper helper = MoreObjects.toStringHelper(this).add("type", type);
        if (comparator != null)
        {
            helper.add("comparator", comparator);
        }
        return helper.toString();
    }

    /**
     * Returns an empty mutable map whose keys will respect this {@link ElementOrder}.
     */
    <K extends T, V> Map<K, V> createMap(int expectedSize)
    {
        switch (type)
        {
            case UNORDERED:
                return Maps.newHashMapWithExpectedSize(expectedSize);
            case INSERTION:
            case STABLE:
                return Maps.newLinkedHashMapWithExpectedSize(expectedSize);
            case SORTED:
                return Maps.newTreeMap(comparator());
            default:
                throw new AssertionError();
        }
    }

    @SuppressWarnings("unchecked")
    <T1 extends T> ElementOrder<T1> cast()
    {
        return (ElementOrder<T1>) this;
    }
}
